Pan detector for induction heating cooking unit

ABSTRACT

A pan detection and control arrangement is provided for an induction heating cooking unit by providing a reed switch and a pair of permanent magnets disposed relative to the reed switch to hold the reed switch in a closed position in the absence of a sufficient size cooking vessel in position overlying the work coil, the reed switch operating to an open position to permit energization of the work coil when the ferromagnetic mass provided by the cooking vessel is in place to shunt a sufficient part of the magnetic field through the vessel and away from the reed switch. Opening of the reed switch permits energization of the work coil.

[ Mar. 12, 1974 PAN DETECTOR FOR INDUCTION 2,497,753 2/1950 Arnot219/1049 HEATING COOKING UNIT 3,426,166 2/1969 Canceill 335/205 [75]Inventors: William C. Moreland, 11, Export;

Robert A. McCoy, Turtle Creek; Primary ExaminerBruce A. Reynolds TerenceD. Malarkey, Monroeville, Attorney, Agent, or FirmE. C. Arenz all of Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa. [57] ABSTRACT [22] Filed: May 1973 A pan detection andcontrol arrangement is provided [21 L N 365,671 for an induction heatingcooking unit by providing a reed switch and a pair of permanent magnetsdisposed relative to the reed switch to hold the reed switch in a [52]US. Cl 219/1049, 219/10.75, 219/518, closed position in the absence of asufficient size 335/219 cooking vessel in position overlying the workcoil, the lift. C1. reed switch operating to a p p i to permit [58] Flldof Search 219/10.49, 10.75, 10.77, energization of the work coil whenthe ferromagnetic 219/518, 519; 335/205, 219, 286; 338/12; mass providedby the cooking vessel is in place to 336/1316" 2; 318/128 shunt asufficient part of the magnetic field through the vessel and away fromthe reed switch, Opening of [56] References and the reed switch permitsenergization of the work coil.

UNITED STATES PATENTS 3,710,062 1/1973 Peters 219/1049 5 Claims, 5Drawing Figures 7 i i 22 g 1 24\ 4 t \J 26'- b0 /-28 l\ L L ":T I J T ,q

PATENTEDHARIZ I91 3.796350 SHEET 2 OF 2 PHASE POWER LINE I gggfig' -gOSCILLATOR i 44 48 i I l MANUAL CONTROL FIG. 5.

PAN DETECTOR FOR INDUCTION HEATING COOKING UNIT CROSS REFERENCES TORELATED APPLICATIONS US. patent application Ser. No. 243,715 and thecompanion US. patent applications referred to therein disclose circuitarrangements with which the arrangement according to the presentinvention may be used.

BACKGROUND OF THE INVENTION 1. Field of the Invention:

The invention pertains to the art of pan detection arrangements for usein induction heating cooking appa ratus.

2. Description of the Prior Art:

It has been suggested heretofore, as in US. Pat. No. 2,497,753, thatproperly locating a mass to be heated by induction heating apparatus canbe used to control a switch controlling the energization of theinduction heating apparatus. In the noted patent, the presence of apressing iron, or a cooking vessel, in a proper location results in theattraction and movement of a permanent magnet toward the mass to therebymove means for closing a switch to permit heating of the iron or vessel.In the arrangement of that patent, the permanent magnet and switch isdisclosed as being located to the side of the pressing iron to be heatedby the induction heating apparatus. Such an arrangement would beinconsistent with the provision of a smooth top cooking platform such asis currently preferred for induction heating cooking apparatus. Thegeneral concept of that arrangement could be employed in connection withcurrent induction heating cooking apparatus by locating a similararrangement in the core of the work coil of the apparatus. However suchan arrangement would have several disadvantages with respect to thearrangement according to the invention. Among these disadvantages arethat the magnet in moving would likely make an audible sound at the endof its travel, the magnet so located would be more sensitive toconcentrated magnetic masses than to a distributed one, the spaceprovided by the core of the work coil is located in a position making ithighly useful for other devices such as a thermal sensor, and themechanism would be exposed to accidental mishandling by servicepersonnel.

An arrangement according to the present invention is considered toobviate all of these objections.

SUMMARY OF THE INVENTION In accordance with an arrangement according tothe invention, there is provided safety control means responsive tolevels of magnetic flux above and below the given range of values forpreventing and permitting, respectively, energization of a work coil,permanent magnet means is disposed relative to the safety control meansand to the location of the base wall of a cooking vessel properlylocated over the work coil to direct a magnetic field with a flux levelabove the given values through the safety control means in the absenceof a ferromagnetic mass at least comparable to the smallest cookingvessel to be used on said coil and in the proper location, with themagnetic field being shunted sufficiently in the presence of the mass inthe proper location to reduce the flux level of the field through thesafety control means to a value below the given values so that unless anadequate load for coupling is present above the coil, the coil isprevented from being energized.

In the currently preferred form of the invention, the safety controlmeans takes the form of a reed switch located generally in a fringemagnetic field between a pair of permanent magnets which have theirpoles arranged and are spaced apart such that in the absence of a masscorresponding to a properly located cooking vessel the flux levelpassing through the reed switch causes it to be closed and in thepresence of the mass the field is shunted sufficiently that the reedswitch opens.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation in thenature of a side elevation showing flux paths in the absence of amagnetic material vessel in position on a work coil;

FIG. 2 is a schematic representation, also in the nature of a sideelevation, showing the shunting effect of the magnetic material vesselon the flux paths;

FIG. 3 is a view in the nature of a vertical cross section showing therelationship between the cooking vessel on the cooking surface, the workcoil, and the sensing assembly;

FIG. 4 is a plan view ofa single cooking unit showing the location ofthe permanent magnets and the reed switch relative to the cookinglocation; and

FIG. 5 is a block diagram and partly schematic view of the .way in whichthe arrangement according to the invention may be connected to a circuitfor controlling the cooking unit.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, the safetycontrol means is shown in its currently preferred form as a sealed,magnetic reed switch 10 having leaf contacts 12 and 14 which extend outthrough the ends of the glass envelope to provide terminals. Thecontacts of the reed switch are normally open but are shown as closed inFIG, 1 becuase of the field from the permanent magnets causes sufficientflux to pass through the leaves of the reed switch to force theircontact ends to touch each other. The magnetic lines of force, some ofwhich are collectively identified by the numeral 16, are produced by thepair of permanent magnets 18 and 20 which have their poles arranged asshown to provide the magnetic field schematically illustrated. Thus theorientation of the poles of the left magnet 18 is with north at the topand south at the bottom, while the orientation of the right magnet isreversed.

FIG. 2 shows the influence upon the location of the lines of force 16exerted by placing an extended area mass 22 of magnetic material inproximity to the reed switch 10. As there shown, the extended area massof magnetice material which corresponds to the bottom wall of a cookingvessel shunts an adequate portion of the field so that the inducedmagnetism in the leaves 12 and 14 is insufficient to maintain the switch10 is a closed position.

Incorporation of the arrangement according to the invention in aninduction heating cooking unit is best understood in connection withFIGS. 3 and 4. A refractory material such as a glass ceramic sheet 24provides a cooking surface above the work coil 26. A suitable I workcoil may comprise litz wire which is wound in a spiral and then moldedin a rubber compound to hold the wire in place with the proper spacingbetween successive convolutions. The work coil also typically includesan underlying layer 28 of the rubber compound; The work coil issupported from below by an underlying Transite sheet or block 30 whichsupports the work coil. The Transite block 30 is supported by means notshown herein. A slot 32 is grooved out of a generally central part ofthe block 30 to receive the reed switch which is held in place bypotting it in with a rubber compound such as Dow Corning Sylgard 185.The groove is made sufficiently long to accommodate the projectingterminal ends of the leads, which are connected to two lead wires 34 and36 (Fig. 3).

In the currently preferred form of the invention, the reed switch shouldbe sufficiently long that the requisite sensitivity is provided withrespect only to magnetic objects having a sufficiently extended area asto be comparable to the smallest diameter cooking vessel intended foruse for the coil. Reed switches which satisfy this requirement are abouttwo to three inches long (including their terminal ends) such as theHamlin Company DRS-Z and DRT-S. Examples of permanent magnets 18 andwhich I have found to function satisfactorily are Allegheny Generallndox V magnets having top and bottom face areas of about 2 inches by 1inch, and about inch thick. Such magnets are adequate for operating reedswitches with closing requirements of 70 ampere-turns and opening valuesof 50 or more ampere turns and are not heated appreciably by the coil ofthe range. The spacing between the facing ends of such magnets used withthe identified reed switch is about 4 inches. However, since variationsmay occur between one reed switch and another, and between one pair ofmagnets and another current practice is to locate the magnets relativeto the reed switch by first bringing them together sufficiently closethat the contacts close, and then bringing a magnetic mass comparable toa cooking vessel bottom into the proper location, then moving themagnets apart until the contacts open. The magnets are then fixed inplace and the operation checked. It is noted in this respect that adifferential may typically be found to exist between the levels ofmagnetic flux effecting closing and opening of the reed switch. Thus itis responsive to levels of magnetic flux above and below a given pair ofvalues, rather than a specific given value.

As may be seen from FIG. 4, it is not necessary that the reed switch andmagnet be centered exactly relative to the center of the work coil. Itis only necessary that it be responsive to the bottom wall of thecooking vessel which is placed generally coincident with the work coil,the location of which is generally indicated by the locating indicia 38provided on the top surface of the cooking surface.

The manner in which the arrangement according to the invention isincorporated in general circuit arrangements of the noted patentapplications for operating an induction heating cooking unit will bedescribed in connection with FIG. 5. The control circuit portion 40 ofthe arrangement disclosed includes a timing and firing capacitor 42 forthe phase controlled rectifier bridge 44. When the pan detector switch10 is open due to the presence of a pan bottom 22, the capacitor chargesup from various sources as disclosed in the noted applications, and isdischarged by a semiconductor switch through a pulse transformer whichcouples into and turns on the phase controlled rectifier bridge.

When the pan detector switch 10 is closed because of the absence of apan, the capacitor 42 is shorted out through resistor 46 and accordinglyprevented from charging up. Hence the phase controlled rectifier bridge44 cannot be turned on, so no power can reach the power oscillator 48and work coil 26.

It will be appreciated that the pan detector switch could beincorporated in the circuit in other ways, such as by controlling arelay which in turn controls power in one part or another of thecircuit.

The arrangement according to the invention provides the following safetyfeatures. It prevents heating of aluminum foil and possible fire hazardsof foil covered packages. It prevents inadvertent heating of metalliccooking utensils such as spoons and forks with consequent burning of theusers hand. lt also protects electronic equipment and pacemaker wearersfrom an energized coil without the shielding provided by a cookingvessel.

It is to be noted that in an arrangement according to the invention theswitch is located in what may be considered to be a fringe portion ofthe magnetic field as distinguished from what could be characterized asa main field if the magnets 18 and 20 were of the type in which thepoles were at the ends of the bars, and the north pole of 18 were facinga south pole of 20. By locating the reed switch in the fringe area ofthe field, placing a relatively extended magnetic mass comprising thebottom wall of the cooking vessel is able to shunt sufficient lines offorce thereto to permit the reed switch to be moved to an open position.

While the currently preferred arrangement includes the use of the reedswitch, it is to be understood that other devices responsive to amagnetic field shunted into and out of position could be usedalternatively, one such example being a Hall generator which could beconnected to provide an on-off switching arrangement.

What we claim is:

1. In an induction heating cooking unit of the type including a workcoil which is adapted to be coupled electromagnetically to a cookingvessel placed in overlying relation to said coil;

safety control means responsive to levels of magnetic flux above andbelow a given range of values of said flux for preventing andpermitting, respectively, energization of said coil;

permanent magnet means disposed relative to said safety control means,and to the location of the base wall of a cooking vessel when properlylocated over said coil, to direct a magnetic field with a flux levelabove said given values through said safety control means in the absenceof a ferromagnetic mass at least comparable to the smallest cookingvessel to be used on said coil and in said proper location;

said magnetic field being shunted sufficiently in the presence of saidmass in said proper location to reduce the flux level of the fieldthrough said safety control means to a value below said given values, sothat unless an adequate load for coupling is present above said, coil,said coil is prevented from being energized.

2. In a unit according to claim ll wherein:

said safety controls means comprises switching means closely adjacentsaid coil; and

3,796,850 5 6 said permanent magnetic means comprise a pair of magneticmass at least comparable to the base wall of permanent magnets onopposite sides of said the smallest cooking vessel to be used with saidcoil switching means, said magnets being disposed relawhen in overlyingrelation to said coil comprising: tive to said switching means, andhaving their poles safety switch means responsive to the presence ofarranged, being spaced apart such that in Said 5 levels of magnetic fluxabove and below a given allsencef of 531d mass the flux f p f l throughrange of values to assume one and another condisaid switching meanscauses sa d switching means on, respectively; tolassume one corldmon andh presence 9 permanent magnet means disposed relative to said salfj lthe field shunted sufficlemly from f safety switch means and to thelocation of the base swltchlng means that the flux levfzl through l wallof the cooking vessel properly located over sw tch ng means is reducedsufficiently that said Said coil, to place said Safety switch means in a3 l meanzassumei i l l magnetic field providing a level of magnetic fluxI accfr to C d d above said given range of values in the absence of l isy con r0 means me u as a mdgne 1C ree said base wall of said cookingvessel, said permal5 nent magnet means being so disposed that suffi- 4.In a unit according to claim 2 wherein: cient lines of force are shuntedsufficiently in the said permanent magnets have their poles oriented f nf It generally vertically, with the north pole of one g gf l i f fi 9 Eg 3 f? uce e eve so magne ic uxmw ic sai sa e y magnet being upwardlyfacing and the north ole of the other magnet being downwardly facingpandswitch means is present to a level below said given with said magnetsbeing spaced apart sufficiently range of f l and that said reed switchis located in a fringe area of elecmcfil f l mefms f controllmgenerglzancf" the magnetic lines f f of said am] including said safetyswitch means in 5. In an induction heating cooking unit of the type inlone n preventmg finerglzatlofl of 531d cluding a work coil underlying arefractory material 11 and In sa d another 00116117410" Permlmng ener'sheet f rmi a ki surface upon hi h a ki gization of said coil, so thatunless an adequate load vessel is adapted to be supported andelectromagnetf oup ing i Pr in h proper loc ion O erically coupled tothe coil when the coil is energized, an lying said coil, said coil isprevented from being enarrangement for controlling the energization ofsaid ergized. work coil in accordance with the presence of a ferro-

1. In an induction heating cooking unit of the type including a workcoil which is adapted to be coupled electromagnetically to a cookingvessel placed in overlying relation to said coil; safety control meansresponsive to levels of magnetic flux above and below a given range ofvalues of said flux for preventing and permitting, respectively,energization of said coil; permanent magnet means disposed relative tosaid safety control means, and to the location of the base wall of acooking vessel when properly located over said coil, to direct amagnetic field with a flux level above said given values through saidsafety control means in the absence of a ferromagnetic mass at leastcomparable to the smallest cooking vessel to be used on said coil and insaid proper location; said magnetic field being shunted sufficiently inthe presence of said mass in said proper location to reduce the fluxlevel of the field through said safety control means to a value belowsaid given values, so that unless an adequate load for coupling ispresent above said coil, said coil is prevented from being energized. 2.In a unit according to claim 1 wherein: said safety controls meanscomprises switching means closely adjacent said coil; and said permanentmagnetic means comprise a pair of permanent magnets on opposite sides ofsaid switching means, said magnets being disposed relative to saidswitching means, and having their poles arranged, and being spaced apartsuch that in said absence of said mass the flux level passing throughsaid switching means causes said switching means to assume onecondition, and in the presence of said mass the field is shuntedsufficiently from said switching means that the flux level through saidswitching means is reduced sufficiently that said switching meansassumes another condition.
 3. In a unit according to claim 1 wherein:said safety control means includes a magnetic reed switch.
 4. In a unitaccording to claim 2 wherein: said permanent magnets have their polesoriented generally vertically, with the north pole of one magnet beingupwardly facing and the north pole of the other magnet being downwardlyfacing, and with said magnets being spaced apart sufficiently that saidreed switch is located in a fringe area of the magnetic lines of force.5. In an induction heating cooking unit of the type including a workcoil underlying a refractory material sheet forming a cooking surfaceupon which a cooking vessel is adapted to be supported andelectromagnetically coupled to the coil when the coil is energized, anarrangement for controlling the energization of said work coil inaccordance with the presence of a ferromagnetic mass at least comparableto the base wall of the smallest cooking vessel to be used with saidcOil when in overlying relation to said coil comprising: safety switchmeans responsive to the presence of levels of magnetic flux above andbelow a given range of values to assume one and another condition,respectively; permanent magnet means disposed relative to said safetyswitch means and to the location of the base wall of the cooking vesselproperly located over said coil, to place said safety switch means in amagnetic field providing a level of magnetic flux above said given rangeof values in the absence of said base wall of said cooking vessel, saidpermanent magnet means being so disposed that sufficient lines of forceare shunted sufficiently in the presence of said base wall of a cookingvessel to reduce the levels of magnetic flux in which said safety switchmeans is present to a level below said given range of values; andelectrical circuit means for controlling energization of said coilincluding said safety switch means in said one condition preventingenergization of said coil and in said another condition permittingenergization of said coil, so that unless an adequate load for couplingis present in the proper location overlying said coil, said coil isprevented from being energized.